Hardened high-contrast photographic silver chloride emulsions and method of processing



United States Patent HARDENED HIGH=ONTRAST PHOTOGRAPHIC SILVER CHLORIDEEMULSIONS AND METHOD OF PROCESSHNG Richard W. Helm and Charles A. Gofie,Rochester, N.Y., assignors to Eastman Kodak Company, Rochester, N.Y., acorporation of New Jersey No Drawing. Filed July 2, 1958, Ser. No.746,079

2 Claims. (Cl. 9627) This invention relates to photography, and moreparticularly, to high-contrast silver chloride emulsions and a method ofprocessing the same.

In the graphic arts field, where it is desired to make photographicreproductions of line and half-tone material, it is customary to employemulsions having extreme contrast. For this purpose, photographic silverchlorobromide emulsions are generally employed, the silver chloridecontent of the emulsions being at least about 60 mole percent. Afterexposure of the emulsions in the conventional way, either to a half-tonenegative, or to an original through a vacuum contact screen, theemulsions are developed in a photographic developer having very lowfreesulfite content. Development is believed to proceed catalytically,and such development is commonly referred to in the graphic arts fieldas infectious development. In order to control the amount offree-sulfite in the developer, it is customary to employ sodiumformaldehydebisulfite in the developer. This material then releasessmall amounts of sulfite to at least partially stabilize the developingsolutions for short periods of time.

However, it has been found that developers containing sodiumformaldehydebisulfite give results which are often erratic, dependingupon the time of standing before use, exposure to air, etc.

Previously, it has been generally thought that the abovementionedcatalytic development can only be obtained if the concentration of thesulfite ion is quite low. See J. A.C., Yule, Journal of the FranklinInstitute, vol. 239, 1945, pages 221-230. It has been thought that thefunction of the sodium formaldehydebisulfite is to provide only verysmall amounts of the sulfite ion and prevent a loss of the high-contrasteffect.

We have now found that certain photographic gelatinosilver-chlorideemulsions containing at least 70 mole percent of silver chloride andcontrolled amounts of free formaldehyde can be processed to provideacceptable halftone reproductions even though processing is carried outin developers having large amounts of the sulfite ion present. Thus, itis possible to use photographic developers containing large amounts ofsulfite without meeting storage difiiculties which accompany the use ofphotographic developers containing only small amounts of sulfite. Thephotographic developer generally employed in our developing compositionsis a hydroquinone compound.

It is, therefore, an object of our invention to provide high-contrastphotographic-gelatino-silver-chloride emulsions containing at least 70mole percent of silver chloride and a predetermined amount of uncombinedformaldehyde. Another object is to provide photographic elementscontaining the above-described silver chloride emulsions, wherein thedried coatings contain a predetermined amount of uncombinedformaldehyde. Still another object is to provide a method of processingsaid silver chloride emulsions to provide high-contrast images usingphotographic developers which can contain large amounts of the sulfiteion. Another object is to provide developers for graphic arts use whichneed not contain sodium formaldehydebisulfite. Other objects will becomeapparent from a consideration of the following description and examples.

3,128,180 Patented Apr. 7, 1964 We have found that the addition ofsufficient formaldehyde to the above-described silver chlorideemulsions, so that the dried coatings contain at least 6 mg./ft.uncombined formaldehyde, provides a means of obtaining acceptablegraphic arts images, even though the emulsions are processed in aphotographic developer containing large amounts of antioxidant, such assodium sulfite.

The silver chloride emulsions employed in our invention can contain from70 to 100 mole percent of silver chloride. We have found that emulsionscontaining from 8 to 30 mole percent of silver bromide and from 70 to 92percent of silver chloride are quite useful in our invention. We havefurther found that the silver halide emulsions can contain a smallamount of silver iodide, such as 1 to 2 mole percent. Of course, it isto be understood that the silver halide emulsions always contain atleast about 70 mole percent of silver chloride.

The amount of formaldehyde added to the silver chloride emulsions canvary, depending upon the degree of finishing, time of drying, etc. Sinceour invention uses gelatin emulsions, it is apparent that a portion ofthe formaldehyde will react with the gelatin to produce a degree ofhardening, depending upon the amount of formaldehyde reacted or lost ondrying. The amount of formaldehyde added to the wet emulsions can varyfrom about 0.1 to 2.0 moles per mole of silver present. The amount ofuncombined formaldehyde remaining in the emulsions after drying willthus be at least 6 mg./ft.

The developers employed in our invention contain a hydroquinone compoundas the active developer or developing agent. Such compounds includehydroquinone, isopropylhydroquinone, methylhydroquinone,2,5-dimethylhydroquinone, etc.' The developer generally can contain fromabout 12 to 20 g./ liter of the hydroquinone developing compound. Thedeveloper will also contain an antioxidant, such as sodium sulfite,S-hydroxyquinoline, ascorbic acid, phloroglucinol,S-hydroxyethylhydrazine,

aminophenols (e.g., N-methyl-p-aminophenol, etc.), metasubstitutedpolyphenols, pyrazolidones (e.g., 3-phenylpyrazolidone, etc.), etc.Sodium sulfite is a particularly useful antioxidant, and we have foundthat from about 0.02 to 0.2 mole of sodium sulfite is suificient tostabilize the developer in the absence of other antioxidants. Ifdesired, a mixture of the afore-mentioned antioxidants can be employed.Suflicient alkaline material should be employed to produce a pH of atleast 9.0, and we have found that from about 70 to g./liter of sodiumcarbonate monohydrate and 10 to 50 g./liter of sodium bicarbonate arequite adequate. Other inorganic alkaline materials, such as sodiumhydroxide, sodium acetate, etc., can be employed, if desired. Thedeveloper should also contain a small amount of a restrainer, such aspotassium bromide, sodium bromide, etc. Generally, about 3 to 5 g./literof restrainer are quite adequate. If desired, the developer can alsocontain other addenda, such as sequestering agents, e.g., tetrasodiumpyrophosphate, etc. The developer can also contain sodiumformaldehydebisulfite, if desired. Generally, from about 50 to 70 g./liter of sodium formaldehydebisulfite is quite adequate. If it isdesired to make a developer suitable for dry packaging, generally nosodium formaldehydebisulfite should be employed unless the developer ispackaged as two separate units, with the sodiurn formaldehydebisulfitein one package and the alkaline material in a separate package.

The silver chloride emulsions of our invention provide a gamma, in someinstances, as high as 11 to 14. The contrast gradient is sometimes ashigh as 9.0 to 9.2. The contrast gradient is a figure obtained from thecharacteristic curve of the emulsion by drawing a line from a point onthe curve 0.10 density above gross fog to a point on the curve 0.4 log Eunits to the right of the first point. Thus, the contrast gradient is agradient over a 0.4 log E exposure range, and includes a part of the toeof the curve.

The effectiveness of the new photographic elements of our invention andtheir use in combination with the abovedescribed developers can bemeasured by exposing the emulsions to a halftone image. After developingthe exposed element, the pattern consists of areas commonly referred toas shadow dots and areas known as highlight areas. Intermediate betweenthese two extremes are areas of varying size. The common method in theart for measuring the quality of the reproduction is to examine theshadow dot areas and the area referred to as 50% dots (i.e., /2 clearand /2 developed density). The quality of the shadow dots can beexpressed on any convenient scale, but for the purposes of ourinvention, we prefer to grade on a scale where a is excellent and g isextremely poor, with varying degrees of quality proportionately gradedbetween these two points. The quality of the 50% dots can convenientlybe expressed in a progressive scale where 9 is excellent and 1 isextremely poor. A reproduction graded 6c is considered acceptable in dotquality. A shadow dot quality below c is generally not acceptable and a50% dot quality below 6 is generally not acceptable.

The following examples will serve to illustrate further the advantagesobtained accor ing to our invention. In these examples, theuncombined-formaldehyde content is expressed as 1X, 4X, 8X or 16X. 1X isconsidered a normal or customary amount and is approximately 2 mg./ft.while 4X is 6 mg./ft. 8X is 8 rng./ft. and 16X is 112 mg./ft. Thedevelopers used in the following examples had the following composition:

TABLE I Developer Compositions [Grams per liter] A B C D E FHydroquinone 15.0 15.0 15.0 15.0 15.0 15.0 Sodium formaldehydebisulfite50. 50. 0 50. 0 Sodium sulfite 1. 25 2. 7.0 10.0 Sodium carbonate. 35.035.0 35.0 35.0 35.0 35. 0 Sodium bicarbonate 22. 5 22. 5 22. 5 27. 5 22.5 22. 5 Potassium br0mide 0.75 0.75 0.75 0.75 0.75 0.75 Sodium bisulfite3.0 2.0 B-Hydroxyethylhydrazine Water to make 1 liter 2.0

EXAMPLE 1 A fine-grain photographic-gelatino-silver-chlorobromideemulsion, containing 80 mol percent of silver as the chloride and 20mole percent as the bromide, was prepared according to the methoddescribed in MacWilliam U.S. Patent 2,756,148, issued July 24, 1956. Toseparate batches of the emulsion was added formaldehyde to give anuncombined formaldehyde content as outlined in Table II. The emulsionscontaining the uncornbined formaldehyde were then coated on aconventional film support, such as cellulose acetate or polystyrene, andthe coatings dried in the usual manner. The dried coatings were thenexposed, using tungsten illumination, to a step wedge through a magentacontact screen of the type described in How to Use the Kodak MagentaContact Screen, Kodak Pamphlet No. Q21, 1956. The exposed coatings werethen developed in the developer composition identified in the table andfixed in a conventional thiosulfate fixing bath.

The contrast gradient, as defined above, dot quality and degree ofswelling of the coatings were then measured. The results are given inthe following table. Also given in the table is the tray life for eachof the developer compositions.

4 TABLE 11 Efiect of Hardener Content EXAMPLE 2 A fine-grainphotographic-gelatino-silver-chlorobromide emulsion was prepared asdescribed in US. Patent 2,756,148, the emulsion containing mole percentof the silver as chloride and 20 mole percent as the bromide. Theemulsion was then treated with suflicient formaldehyde so that the driedphotographic element contained 8 mg./ft. of uncombined formaldehyde. Theemulsion was then coated on a conventional film support and dried. Thedried coatings were then exposed through a magenta contact screen to astep wedge as in Example 1 above. In order to measure the relative speedof the coatings, only one-half of the surface was covered by the contactscreen. Separate strips of the coating were then developed in DeveloperA of Table I to which the addendum had been added, as identified in thefollowing table. The strips were developed for the times shown in thetable, both with fresh developer and developer which had been aeratedfor six hours in an 8 x 10-inch tray containing 500 cc. of developer at68 F. The strips were then fixed in a thiosulfate fixing solution.Contrast and dot quality of the area under the contact screen weremeasured and the speed of the emulsion was determined from the area ofthe coating not in contact wtih the magenta contact containing molepercent of the silver as chloride, and 10 mole percent as the bromide,was prepared according to the method of Us. Patent 2,756,148. Theemulsion was then sensitized with a sulfur sensitizer, as disclosed inSheppard U.S. Patent 1,574,944, issued March 2, 1925, and with a goldsalt, as described in Damschroder U.S. Patent 2,597,856, issued May 27,1952, or Yutzy et al. US. Patent 2,597,915, issued May 27, 1952. Theemulsion was then divided into two separate batches. One batch wastreated with suflioient formaldehyde to produce a normal content ofuncombined formaldehyde (IX), while the other batch was treated withsufiicient formaldehyde to produce 16X unconrbined formmdehyde. Each ofthe batches was coated on separate film supports, as described above,and dried. The dried coatings were then exposed to a step wedge througha magenta contact screen, as described above, and developed in DeveloperD of Table I to which sodium sulfite had been added in the amounts shownin the table. After fixing in the usual manner, the contrast gradientand dot quality (shadow) Hafdener and Sulfite Variations 16X Hardener inEmulsion 1X Hardener in Emulsion Sodium Sulflte (to Developer D) (Gramsper Liter) Dot Quality Dot Contrast Quality Contrast EXAMPLE 4 Separatebatches of gelatino-silver-halide emulsions having the compositionsshown in the following table were prepared and the emulsions thensensitized with a sulfur compound, as described in U.S. Patent1,574,944. The emulsions were then coated on separate film supports anddried. The dried coatings were exposed to a step wedge through a magentacontact screen, which covered only half of the coating so that theemulsion speed could be determined. The coatings were developed for theshown in the following table, using the developers identified in thetable. A separate series of strips were then processed in an identicalmanner, except that the developers were aerated for 6 hours, asdescribed in Ex ample 2 above. Each set of strips was fixed and dried.The contrast and dot quality for each of the coatings are shown, as wellas the development latitude to obtain optimum dot quality. The emulsionspeeds shown in the table are relative. The results obtained were asfollows:

TABLE V Conventional vs. Improved Systems the hydroquinone developer andsodium formaldehydebisulfite are in one package, while the alkalineingredients are present in a separate package. Thus, Developers A, B andC, for example, should be mixed immediately prior to use. Even smallquantities of antioxidants have been shown to provide useful results inextending the life of the developers, as can be see-n by reference tothe above tables. As indicated above, the pH of the developer shouldgenerally be at least about 9.0, although we have found thatparticularly good results are obtained in those cases where the pH is atleast about 9.6.

As indicated above, the photographic elements prepared acoording to ourinvention should contain at least about 6 mg/ft. of uncombinedformaldehyde. in the coatings described in Examples 1-4 above, thephotographic emulsions contained about 615 mg. of gelatin, 540 mg. ofsilver and 11.5 mg. of saponin (coating aid) per square foot. The amountof formaldehyde added to these coatings, to obtain the degree ofhardness indicated in the examples, is indicated in the followingtabulation, which compares the uncombined formaldehyde in the freshcoating before drying with the uncombined formaldehyde after drying.

The photographic silver chloride emulsions prepared according to ourinvention are useful in preparing halftone negatives from continuoustone originals without using a half-tone screen. For instance, if nosodium Emulsion Composition D Efiect of Aeration ev. Devel- Time Con-Emul. Dot Lat.

Ohlo- Bro- Hardoper (Min) trast Speed Quality (Min) Emul- Conride mideIodide ener siou trast Speed 70 30 0 1X A 2% 6. 2 100 7c 1%; 28 2. 5

90 9 1 16X F 2% 7. 3 590 7b 24% 690 8. 6

The above table shows that the hardened chlorobromide hyde.

The silver chloride emulsions used in our invention can conveniently beortho-sensitized or pan-sensitized with spectral sensitizing dyes. Forinstance, these emulsions can be spectrally sensitized by treating witha solution of a sensitizing dye in an organic solvent. sensitizing dyesuseful in sensitizing the emulsions of our invention are described inBIDlOkGI et a1. U.S. Patent 2,526,632, issued October 24, 1950, SpragueU.S. Patent 2,503,776, issued April 11, 1950, and others.

The developers described in Table I above vary in their stability, asshown in the above examples. Developer E was found to provide goodresults during processing even after storage for 12 months in solutionand showed only a 0.15 log E shift in speed, as compared with freshdeveloper. On the other hand, Developer A, stored under the sameconditions, showed loss of activity in 24 hours and sulfite is employedin the developing compositions and the coatings are exposed to acontinuous tone original, small, round spots appear in the reproductionwhich can be conveniently called pepper spots. These spots correspondroughly to a half-tone pattern. However, the results ob tained from thehardened emulsions of our invention are particularly outstanding whenemployed in combination with developers containing at least 0.01 mlolarsulfite ion, such as Developer B above.

The above coatings were dried for 4 to 6 minutes in a chamber having adry bulb temperature of F. and a wet bulb temperature of 58 F.

The term uncombined formaldehyde as used herein means formaldehydedetectable by a chemical analysis, such as that of SB. Schryver, Proc.Royal Soc, B. vol. 82, page 226 (1910).

The term half-tone as used in the foregoing description and in thefollowing claims means not only conventional dot patterns but also thewell-known line patterns. That is, line screens can also be employedaccording to the above-described process to provide the unusual effectsillustrated.

The photographic emulsions used in practicing our invention are, ofcourse, of the developing-out type.

The emulsions can be chemically sensitized by any of the acceptedprocedures. The emulsions can be digested with naturally active gelatin,or sulfur compounds can be added such as those described in SheppardU.S. Patent 1,574,944, issued March 2, 1926, and Sheppard et al. U.S.Patent 1,623,499, issued April 5, 1927, and Sheppard and Brigham U.S.Patent 2,410,689, issued November 5, 1946.

The emulsions can also be treated with salts of the noble metals such asruthenium, rhodium, palladium, iridium, and platinum. Representativecompounds are ammonium chloropalladate, potassium chloroplatinate, andsodium chloropalladite, which are used for sensitizing in amounts belowthat which produces any substantial fog inhibition, as described inSmith and Trivelli U.S. Patent 2,448,060, issued August 31, 1948, and asantifoggants in higher amounts, as described in Trivelli and Smith U.S.Patents 2,566,245, issued August 28, 1951, and 2,566,263, issued August28, 1951.

The emulsions can also be chemically sensitized with gold salts asdescribed in Waller et al. U.S. Patent 2,399,- 083, issued April 23,1946, or stabilized with gold salts as described in Damschroder U.S.Patent 2,597,856, issued May 27, 1952, and Yutzy and Leermakers U.S.Patent 2,597,915, issued May 27, 1952. Suitable compounds are potassiumchoroaurite, potassium aurithiocyanate, potassium chloroaurate, aurictrichloride and 2-aurosulfobenzothiazole methochloride.

The emulsions can also be chemically sensitized with reducing agentssuch as stannous salts (Carroll U.S. Patent 2,487,850, issued November15, 1949), polyamines, such as diethylene triamine (Lowe and Jones U.S.Patent 2,518,698, issued August 15, 1950), polyamines, such as spermine(Lowe and Allen U.S. Patent 2,521,925, issued September 12, 1950), orbis(fi-aminoethyl) sulfide and its water-soluble salts (Lowe and JonesU.S. Patent 2,521,- 926, issued September 12, 1950).

The emulsions can also be optically sensitized with cyanine andmerocyanine dyes, such as those described in Brooker U.S. Patents1,846,301, issued February 23, 1932; 1,846,302, issued February 23,1932; and 1,942,- 854, issued January 9, 1934; White U.S. Patent1,990,507, issued February 12, 1935; Brooker and White U.S. Patents2,112,140, issued March 22, 1938; 2,165,338, issued July 11, 1939;2,493,747, issued January 10, 1950; and 2,739,- 964, issued March 27,1956; Brooker and Keyes U.S. Patent 2,493,748, issued January 10, 1950;Sprague U.S. Patents 2,503,776, issued April 11, 1950, and 2,519,001,issued August 15, 1950; Heseltine and Brooker U.S. Patent 2,666,761,issued January 19, 1954; H'eseltine U.S. Patent 2,734,900, issuedFebruary 14, 1956; Van Lare U.S. Patent 2,739,149, issued March 20,1956; and Kodak Limited British Patent 450,948, accepted July 15, 1936.

The emulsions can also be stabilized with the mercury compounds ofAllen, Byers, and Murray U.S. Patent 2,728,- 663, issued December 27,1955; Carroll and Murray U.S. Patent 2,728,664, issued December 27, 1955and Leubner and Murray U.S. Patent 2,728,665, issued December 27, 1955the triazoles of Heimbach and Kelly U.S. Patent 2,444,608, issued July6, 1948; the azaindenes of Heimbach and Kelly U.S. Patents 2,444,605 and2,444,606, issued July 6, 1948; Heimbach U.S. Patents 2,444,607, issuedJuly 6, 1948 and 2,450,397, issued September 28, 1948; Heimbach andClark U.S. Patent 2,444,609, issued July 6, 1948; Allen and ReynoldsU.S. Patents 2,713,541, issued July 19, 1955, and 2,743,181, issuedApril 24, 1956; Carroll and Beach U.S. Patent 2,716,062, issued August23, 1955 Allen and Beilfuss U.S. Patent 2,735,769, issued February 21,1956; Reynolds and Sagal U.S. Patent 2,756,147, issued July 24, 1956;Allen and Sagura U.S. Patent 2,772,164, issued November 27, 1956, andthose disclosed by Birr in Z. wiss. Phot., vol. 47, 1952, pages 2-28;the quaternary benzothiazolium compounds of Brooker and Stand U.S.Patent 2,131,038, issued Septemher 27, 1938; and the zinc and cadmiumsalts of Jones U.S. patent application Serial No. 493,047, filed March8, 1955.

The emulsions may also contain speed-increasing compounds of thequaternary ammonium type of Carroll US. Patent 2,271,623, issuedFebruary 3, 1942; Carroll and Allen U.S. Patent 2,288,226, issued June13, 1942; and Carroll and Spence U.S. Patent 2,334,864, issued November23, 1943; and the polyethylene glycol type of Carroll and Beach U.S.Patent 2,708,162, issued May 10, 1955.

The emulsions may contain a suitable gelatin plasticizer such asglycerin; a dihydroxy alkane such as 1,5-pentane diol as described inMilton and Murray U.S. application Serial No. 588,951, filed June 4,1956; an ester of an ethylene bis-glycolic acid' such as ethylenebis(met'nyl glycolate) as described in Milton U.S. application SerialNo. 662,564, filed May 31, 1957; bis-(ethoxy diethylene glycol)succinate as described in Gray U.S. application Serial No. 604,333,filed August 16, 1956, or a polymeric hydrosol as results from theemulsion polymerization of a mixture of an amide of an acid of theacrylic acid series, an acrylic acid ester and a styrene-type compoundas described in Tong U.S. patent application Serial No. 311,319, filedSeptember 24, 1952. The plasticizer may be added to the emulsion beforeor after the addition of a sensitizing dye, if used.

The emulsions may contain a coating aid such as sapomin; a lauryl oroleoyl monoether of polyethylene glycol as described in Knox and DavisU.S. Patent 2,831,766, issued April 22, 1958; a salt of a sulfat'ed andalkylated polyethylene glycol ether as described in Knox and Davis U.S.Patent No. 2,719,087, issued September 27, 1955; an acylated alkyltaurine such as the sodium salt of N-oleoyl-N-methyl taurine asdescribed in Knox, Twardokus and Davis U.S. Patent 2,739,891, issuedMarch 27, 1956; the reaction product of a dianhydride oftetracarboxybutane with an alcohol or an aliphatic amine containing from8 to 18 carbon atoms which is treated with a base, for example, thesodium salt of the monoester of tetracarboxybutane as described in Knox,Stenberg and Wilson U.S. patent application Serial No. 485,812, filedFebruary 2, 1955; a water-soluble maleopimarate or a mixture of awater-soluble maleopimarate and a sub,- stituted glutamate salt asdescribed in Knox and Fowler U.S. Patent 2,823,123, issued February 11,1958; an alkali metal salt of a substituted amino acid such as disodiumN-(carbo-p-tert. octylphenoxypentaethoxy)glutamate as described in Knoxand Wilson U.S. patent application Serial No. 660,679, filed July 30,1956; or a sulfosuccinamate such as tetrasodium N-(1,2-dicarboxyethyl)-N-octadecyl sulfosuccinamate or N-lauryl disodium sulfosuccinamate asdescribed in Knox and Stenberg U.S. patent application Serial No.691,125, filed October 21, 1957.

In the preparation of the silver halide dispersions employed forpreparing silver halide emulsions, there may be employed as thedispersing agent for the silver halide in its preparation, gelatin orsome other colloidal material such as colloidal albumin, a cellulosederivative, or a synthetic resin, for instance, a polyvinyl compound.Some colloids which may be used are polyvinyl alcohol or a hydrolyzedpolyvinyl acetate as described in Lowe U.S. Patent 2,286,215, issuedJune 16, 1942; a far hydrolyzed cellulose ester such as celluloseacetate hydrolyzed to an acetyl content of 1926% as described in U.S.Patent 2,327,808 of Lowe and Clark, issued August 24, 1943; awater-soluble ethanolamine cellulose acetate as described in Yutzy U.S.Patent 2,322,085, issued June 15, 1943; 21 polyacrylamide having acombined acrylamide content of '3060% and a specific viscosity of 025-15on an imidized polyacrylamide of like acrylamide content and viscosityas described in Lowe, Minsk and Kenyon U.S. Patent 2,541,474, issuedFebruary 13, 1951; zein as described in Lowe U.S. Patent 2,563,791,issued August 7, 1951; a vinyl alcohol polymer containing urethanecarboxylic acid groups of the type described in Unruh and Smith US.Patent 2,768,154, issued October 23, 1956; or containing cyano-acetylgroups such as the vinyl alcohol-vinyl cyanoacetate copolymer asdescribed in Unruh, Smith and Priest US. Patent 2,808,331, issuedOctober 1, 1957; or a polymeric material which results from polymerizinga protein or a saturated acylated protein with a monomer having a vinylgroup as described in US. application Serial No. 527,872 ofIllingsworth, Dann and Gates, filed August 11, 1954.

If desired, compatible mixtures of two or more of these colloids may beemployed for dispersing the silver halide in its preparation.Combinations of these antifoggants, sensitizers, hardeners, etc., may beused.

The invention has been described in detail with par= ticular referenceto preferred embodiments thereof, but it will be understood thatvariations and modifications can be efiected Within the spirit and scopeof the invention as described hereinabove and as defined in the appendedclaims.

What we claim as our invention and desire secured by Letters Patent ofthe United States is:

1. In the preparation of a half-tone pattern by exposing a dried, thinphotographic silver halide emulsion layer on a photographic support to ahalf-tone image and developing the resulting exposed silver halideemulsion layer with a photographic aqueous developer, the improvementwhich comprises employing a high-contrast photographic gelatino-silverhalide emulsion layer containing at least 6 mg. per square foot ofuncombined formaldehyde, the quantity of formaldehyde present in saidsilver halide emulsion layer prior to drying being between about 0.1 and2.0 mole per mole of silver halide, said silver halide emulsioncontaining at least 70 mole percent silver chloride, and a photographicaqueous developer having a pH of at least 9.0 and containing ahydroquinone as the active developing agent and at least 0.01 molar ofsulfite ion.

2. In the preparation of a half-tone pattern by exposing a dried, thinphotographic silver halide emulsion layer on a photographic support to ahalf-tone image and developing the resulting exposed silver halideemulsion layer with a photographic aqueous developer, the improvementwhich comprises employing a high contrast photographic gelatino-silverhalide emulsion layer containing at least 6 mg. per square foot ofuncombined formaldehyde, the quantity of formaldehyde present in saidsilver halide emulsion layer prior to drying being between about 0.1 and2.0 mole per mole of silver halide, said silver halide emulsioncontaining at least mole percent silver chloride, 8 to 30 mole percentsilver bromide and 0 to 2 mole percent silver iodide, and a photographicaqueous developer having a pH of at least 9.6 and containinghydroquinone as the active developing agent, at least 0.01 molar ofsodium sulfite, sodium formaldehyde-bisulfite and potassium bromide as arestrainer.

References Cited in the file of this patent UNITED STATES PATENTS2,309,497 Albers et al. Jan. 26, 1943 2,533,650 Wattier Dec. 12, 19502,585,596 Stanton Feb. 12, 1952 2,586,168 Kaszuba Feb. 19, 19522,652,345 Jones Sept. 15, 1953 2,716,059 Yutzy et al. Aug. 23, 19552,725,295 Allen et al. Nov. 29, 1955 2,754,208 Conrad July 10, 19562,756,148 MacWilliam July 24, 1956 2,845,349 Schwarz July 29, 19582,887,377 Dickerson May 19, 1959 2,902,367 Welliver et al. Sept. 1, 19593,030,209 Henn et al. Apr. 17, 1962 OTHER REFERENCES Yule: Journal ofthe Franklin Institute, vol. 239, 1945, pages 221-230. (Copy in Sci.Library T1F8.)

Mees: The Theory of Photographic Process, 1942, Macmillan Co., NY.(Preface, pages vii-viii.) Copy in Sci. Library TR a14t.

1. IN THE PREPARATION OF A HALF-TONE PATTERN BY EXPOSING A DRIED, THINPHOTOGRAPHIC SILVER HALIDE EMULSION LAYER ON A PHOTOGRAPHIC SUPPORT TO AHALF-TONE IMAGE AND DEVELOPING THE RESULTING EXPOSED SILVER HALIDEEMULSION LAYER WITH A PHOTOGRAPHIC AQUEOUS DEVELOPER, THE IMPROVEMENTWHICH COMPRISES EMPLOYING A HIGH-CONTRAST PHOTOGRAPHIC GELATINO-SILVERHALIDE EMULSION LAYER CONTAINING AT LEAST 6 MG. PER SQUARE FOOT OFUNCOMBINED FORMALDEHYDE, THE QUANTITY OF FORMALDEHYDE PRESENT IN SAIDSILVER HALIDE EMULSION LAYER PRIOR TO DRYING BEING BETWEEN ABOUT 0.1 AND2.0 MOLE PER MOLE OF SILVER HALIDE, SAID SILVER HALIDE EMULSIONCONTAINING AT LEAST 70 MOLE PERCENT SILVER CHLORIDE, AND A PHOTOGRAPHICAQUEOUS DEVELOPER HAVING A PH OF AT LEAST 9.0 AND CONTAINING AHYDROQUINONE AS THE ACTIVE DEVELOPING AGENT AND AT LEAST 0.01 MOLAR OFSULFITE ION.